1. Field of the Invention
This invention relates to a card which makes use of a thermally reversible thermosensitive recording material whereby data information can be rewritten in the card repeatedly (hereinafter referred to simply as "rewriteable card") and also to a printing apparatus for printing the card and a method for judging the life thereof.
2. Description of the Prior Art
In recent years, a specific type of card called rewriteable card has been employed, as a pre-paid card, in the field of various pastimes or services. The rewriteable card ordinarily makes use of a thermosensitive recording material which is a kind of composite material consisting of high molecular weight and low molecular weight materials, by which data information can be erased and rewritten repeatedly.
When the thermally reversible thermosensitive recording material used as the rewriteable card is heated to a predetermined level of temperature, it changes from a transparent state to a slight opaque state. When the material is heated to another level of temperature, it changes from the slightly opaque state to the transparent state. At normal temperature, the material keeps either of the states as it is. In the rewriteable card, printed information can be erased and rewritten a desired number of times by application of appropriate heat energies.
The rewriteable card may comprise a base color layer, such as an aluminum thin sheet, added to an intermediate layer portion of a card structure. By this, the base color of the layer can be seen through the transparent portion of the thermosensitive recording material, thereby increasing a contrast ratio to the slightly opaque portion to make clearer print.
A printing apparatus provided with a thermal head which can print the rewriteable card. Used rewriteable card can be collected and recycled by changing its data information, which is very useful in view of the effective utilization of resources.
However, since the rewriteable card is recycled by repeatedly erasing and rewriting the printed information, the density of slight opacity of the slight opaque portion is gradually lowered as the number of times of the erasing and rewriting of the printed information is increased. As a result, the transparent portion remains on the portion which must be the slight opaque portion originally so that the base color can be seen through the transparent portion, thereby decreasing the contrast ratio between the original slight opaque portion and the base color portion.
If the printed information comprises the one which is visible by human eyes such as letters and/or numerals, the rewriteable card can be used without any trouble even if the contrast ratio is decreased when the base color is black.
However, if the data information comprises the one such as a bar code, etc. which can be recognized by an electro-optical technique, there is a possibility that a reader such as a bar code reader can not read the bar code, etc., or erroneously reads. As a result, there was the possibility that a system employing the bar code, etc., had not functioned as a whole.
The rewriteable card is liable to have a flaw on its surface caused by the contact with the thermal head during printing or by the repeated use thereof, which results in generation of the problem that the printed information such as the bar code, etc., is liable to be defective.